Molecular BIology of Cancer Topics
DNA Repair
The inherited colon cancer syndromes
include Hereditary Nonpolyposis Colon Cancer (HNPCC, aka Lynch Syndrome),
and Familial Adenomatous Polyposis (FAP). FAP accounts for about 1% of colonorectal
cancer cases, occurs with very frequent bening adenomatous polyps (hundresds
per patioent), some of which progress to carcinomas. The main gene involved
is APC.
HNPCC accounts for about 10% of colonorectal cancer cases, occurs with very few bening lesions (adenomas) and the genes involved are hMSH2, hMSH1, and hPMS2. Surprisingly, loss of heterozygosity (LOH) of the marker gene D2S123 (cosegrates with colon cancer) does not occur in HNPCC, and only in a minority of sporadic colon tumors. Most HNPCC tumors exhibit shifts in the electrophoretic mobility of certain dinucleotide repeat fragments, (CA)n, suggesting that replication errors (RER) had occurred during tumor development. Most inherited tumors and some sporadic tumors show RERs, not only for the D2S123 gene, but also for many other markers, even on different chromosomes. Therefore HNPCC patients inherit a mutation in a gene loaclized near D2S123 on chromosome 2 which plays a role in preventing genome-wide shifts in the length of microsatellite dinucleotide repeats, that gives rise to the RER+ phenotype. Patients with sporadic RER+ tumors may have a somatic mutation in the same gene.
Microsatellites occur on the average of once every 100,000 bp of genomic DNA. By extrapolation, the genome of RER+ tumor cells may contain thousands of changes in microsatellite dinucleotide repeat length. Some microsatellites occur in coding sequences. Thus a mutation in the putative HNPCC gene could result in functional mutation of other genes. In other words, a mutation in the HNPCC gene could result in a "mutator phenotype". Several human diseases have been attributed to an expansion of a trinucleotide repeat in the disease-associated gene: Fragile X syndrome, Kennedy’s syndrome, myotonic dystrophy, Huntington’s disease, etc. The mutation can be inherited or somatic.
Studies of DNA mismatch repair in bacteria and yeast provided clues to the molecular defect in HNPCC. The DNA mismatch repair mechanism fixes mismatches of nucleotides due to DNA damage, nucleotide misincorporation during replication, or heteroduplex formation as a result of recombination.
A
well known mismatch repair mechanism is the MutHLS pathway in E. coli. This
pathway depends on the genes mutH, mutS, mutL and mutU (uvrD). mutS protein
binds to the mismatched nucleotides. mutH cuts the DNA strand (long patch,
~ 2 kb excision repair). mutL links mutS and mutH. DNA polymerase and ligase
are also required. Similar repair genes have been identified in yeast. Extracts
of human cells can catalize the same type of repair, suggesting that humans
have a similar DNA mismatch repair system.
Mutations in the mutHLS genes in yeast lead to alterations in length of dinucleotide repeat tracts, much like RER+ tumors in HNPCC patients. Mutations in either mutL genes or mutS genes greately increase the rate of mutations in dinucleotide repeat tracts lengths by several hundredfold. Mutations in DNA polymerase have a much smaller effect (<10 fold).
It seem that DNA polymerase
in vivo has a high rate of error on templates containing simple repeats, but
most of these errors are corrected by the mutHLS mismatch repair system. DNA
polymerase has 3’to 5’ exonuclease activity to correct base misincorporation.
However, if strands of repetitive DNA dissociate and reassociate in a “slipped”
pairing, DNA polymerase is too far away to sense and repair the damage. Since
the MutHLS pathway corrects such heteroduplex mismatches, the HNPCC gene may
be a homologue of mutS, mutL or mutH. RER+ tumors are also defective in
base-base mismatch repair. In E. coli, the same mutHLS system repairs both
base-base and slipped-strand missmatches.
Eventually,
the HNPCC gene on chromosome 2 was identified as hMSH2, a homologue of the
yeast msh2 and E. coli mutS genes. The hMSH2 gene is mutated in HNPCC tumors
and in sporadic colon cancer tumors with the RER+ phenotype. For example in
one of the families studied, 11 affected individuals had the C to T mutation
in codon 622, and 10 unaffected individuals had two normal alleles. Thus there
was a perfect segregation of hMSH2 mutation with HNPCC disease.
